1. Field of the Invention
This invention relates to digital circuits. In particular, the invention relates to impedance control.
2. Description of Related Art
Compensation codes are used to compensate for any impedance mismatches at input/output (I/O) pads of I/O drivers. This is referred to as resistor compensation (RCOMP). Impedance mismatches may be caused by several factors including type of process, junction temperature, die temperature profile, noise, power supply fluctuations, device age, warm-up variations, etc. When a circuit involving I/O drivers is functioning, these factors may affect the impedance of the individual I/O drivers causing undesirable effects such as noise, reflections, cross-talks, ringing, overshoots and undershoots. To maintain reasonable balanced impedance, compensation codes are updated periodically.
Current techniques for updating compensation code are static in that the compensation codes are sent to the I/O drivers at a fixed, predetermined schedule. One approach is to update using a special reserved cycle (SRC). At designated times, the SRC is initiated, the transmission of data on the bus is stopped and the I/O drivers are tri-stated. Then, the updated compensation codes are sent to the I/O drivers. This approach has a number of disadvantages. First, the transmission of data is temporarily suspended, resulting in slow transmission rate. Second, special circuitry is required to initiate the SRC. Third, the core of the device has to have some routing mechanism to route pending transactions into queues prior to code update. This approach therefore results in complex circuitry and degraded performance.